The sample holder is a component of an electron microscope providing the physical support for samples under observation. Sample holders traditionally used for TEMs and STEMs, as well as some modern SEMs, consist of a rod that is comprised of three key regions: the end, the barrel and the specimen tip. In addition to supporting the sample, the sample holder provides an interface between the inside of the instrument (i.e., a vacuum environment) and the outside world.
To use the sample holder, one or more samples are first placed on a sample support device. The sample support device is then mechanically fixed in place at the specimen tip, and the sample holder is inserted into the electron microscope through a load-lock. During insertion, the sample holder is pushed into the electron microscope, assisted by the vacuum within the microscope, until it stops, which results in the specimen tip of the sample holder being located in the column of the microscope. At this point, the barrel of the sample holder bridges the space between the inside of the microscope and the outside of the load lock, and the end of the sample holder is outside the microscope. To maintain an ultra-high vacuum environment inside the electron microscope, flexible o-rings are typically found along the barrel of the sample holder, and these o-rings seal against the microscope when the sample holder is inserted. The exact shape and size of the sample holder varies with the type and manufacturer of the electron microscope, but each holder contains these three key regions.
The sample holder can also be used to provide stimulus to the sample, and this stimulus can include temperature (e.g., heating or cooling), electricity (e.g., applying a voltage or current), mechanical (e.g., applying stress or strain), gas or liquid (e.g., containing a sample in a specific gaseous or liquid environment), or more than one of these at once. For example, a syringe pump can be used to force liquids through a sample holder containing a sample during imaging. This equipment is outside of the microscope, and various connectors are used to bring this stimulus down the length of the holder to the sample holder, and to the samples. For example, microfluidic tubing can be used to supply liquids from the syringe pump to the sample.
One configuration is an environmental cell wherein two semiconductor devices comprising thin windows are used, and samples are sandwiched between the two semiconductor devices, and the environment in proximity of the sample, including an electrical field, temperature, and a gas or liquid flow, can be precisely controlled. The present inventors previously described apparatuses and methods to contact and align devices used to form liquid or gas cells in International Patent Application No. PCT/US2011/46282 filed on Aug. 2, 2011 entitled “ELECTRON MICROSCOPE SAMPLE HOLDER FOR FORMING A GAS OR LIQUID CELL WITH TWO SEMICONDUCTOR DEVICES,” which is hereby incorporated herein in its entirety.
It is advantageous to be able to monitor the conditions of the environment at or near the sample. Conditions of particular interest include temperature, pressure and chemical properties such as pH. Disadvantageously, most traditional sensors for measuring such conditions are too large to incorporate into the relatively small spaces of an electron microscope holder. Although some traditional sensors will allow for the measurements farther away from the sample, this limits the ability to ensure high accuracy and dynamic control. A sensor mounted very close to the sample of the holder is needed. One technology that could allow for localized measurements of environmental properties in small areas is fiber optic sensors. Since some of these sensors are available in diameters less than 150 micrometers, they are small enough to overcome previous challenges and by using a delicate balance of design features and parameters, there are a variety of unique ways these sensors can be assembled into the holders.
Fiber optic sensors operate by transporting light by wavelength or intensity to provide information about the environment surrounding the sensor. The environment surrounding a fiber optic sensor is usually liquid or gas. Fiber optic sensors can be categorized as intrinsic or extrinsic. Extrinsic fiber optic sensors simply use an optical fiber to transport light. An example is the laser induced fluorescence (LIF) cone penetrometer. The optical fiber is only a conduit for the laser induced fluorescence to be transported to an uphole detector. In contrast, intrinsic fiber optic sensors use the fiber directly as the detector.
There are a variety of intrinsic fiber optic sensors that could be used to measure environmental properties at the tip of an environmental TEM holder. These include pressure, temperature and chemical fiber optic sensors.
There are specific advantages to measuring temperature of the gas or liquid environment. One primary advantage is accuracy. By monitoring the temperature of the gas or liquid within close proximity to the microscope sample, heat transfer losses can be minimized by ensuring that the temperature is readjusted in real time to the precise required temperature, which is particularly important if the heat source is located at a relative distance from the sample. Many experiments require an accurate temperature to conduct a successful experiment. For example, live biological samples will die if the liquid temperature is too high and certain electrochemical reactions require a stable temperature.
Measuring the absolute pressure of the gas or liquid at the sample offers many advantages as well. One advantage is safety. By monitoring and controlling the pressure, the closed cell system can be kept at a safe level, e.g., avoiding overpressure which can break thin semiconductor membranes and introduce gas into the column of the electron microscope. A second advantage is the ability to calculate the temperature within a gas environment. Since the actual temperature of any given gas or gas mixture is a function of the pressure, and because convective heat transfer of gas can cool heated objects placed in a gaseous environment as a function of pressure, the gas pressure can be measured to accurately calculate the temperature of the heated object. Thirdly, the reaction rate of many reactions is pressure dependent, and the ability to measure and control the pressure is essential to understanding and analyzing experimental results.
Monitoring chemical properties at the sample, such as pH levels in a liquid environmental cell, enable the user to correlate the value of this property in relationship to a reaction(s) observation. It also allows the user or the system to make adjustments to the composition or metering rate of the fluid flowing to the sample region.
It is therefore an object of the present invention to provide a sample holder and a sensor or sensors to allow the user to accurately and efficiently measure the environmental properties including pressure, temperature and pH on or within the electron microscope holder in proximity of the sample.